diff --git a/PFERD/__main__.py b/PFERD/__main__.py
index 59004ae..26a1dc4 100644
--- a/PFERD/__main__.py
+++ b/PFERD/__main__.py
@@ -119,7 +119,7 @@ def main() -> None:
 
     try:
         loop = asyncio.get_event_loop()
-        loop.run_until_complete(pferd.run())
+        loop.run_until_complete(pferd.run(args.debug_transforms))
         loop.run_until_complete(asyncio.sleep(1))
         loop.close()
     except ConfigOptionError as e:
diff --git a/PFERD/cli/parser.py b/PFERD/cli/parser.py
index e6b0671..269a19a 100644
--- a/PFERD/cli/parser.py
+++ b/PFERD/cli/parser.py
@@ -164,6 +164,11 @@ PARSER.add_argument(
     help="dump current configuration to a file and exit."
     " Use '-' as path to print to stdout instead"
 )
+PARSER.add_argument(
+    "--debug-transforms",
+    action="store_true",
+    help="apply transform rules to files of previous run"
+)
 PARSER.add_argument(
     "--crawler", "-C",
     action="append",
diff --git a/PFERD/crawl/crawler.py b/PFERD/crawl/crawler.py
index 321daa2..aa0f81c 100644
--- a/PFERD/crawl/crawler.py
+++ b/PFERD/crawl/crawler.py
@@ -3,7 +3,7 @@ import os
 from abc import ABC, abstractmethod
 from datetime import datetime
 from pathlib import Path, PurePath
-from typing import Any, Awaitable, Callable, Dict, List, Optional, Sequence, Tuple, TypeVar
+from typing import Any, Awaitable, Callable, Dict, List, Optional, Sequence, Set, Tuple, TypeVar
 
 from rich.markup import escape
 
@@ -334,3 +334,21 @@ class Crawler(ABC):
         """
 
         pass
+
+    def debug_transforms(self) -> None:
+        self._output_dir.load_prev_report()
+
+        if not self.prev_report:
+            log.warn("Couldn't find or load old report")
+            return
+
+        seen: Set[PurePath] = set()
+        for known in self.prev_report.known_files:
+            looking_at = list(reversed(known.parents)) + [known]
+            for path in looking_at:
+                if path in seen:
+                    continue
+
+                log.explain_topic(f"Transforming {fmt_path(path)}")
+                self._transformer.transform(path)
+                seen.add(path)
diff --git a/PFERD/pferd.py b/PFERD/pferd.py
index 7f4d6ff..ac373cf 100644
--- a/PFERD/pferd.py
+++ b/PFERD/pferd.py
@@ -92,7 +92,14 @@ class Pferd:
                 if isinstance(crawler, KitIliasWebCrawler):
                     crawler.share_cookies(kit_ilias_web_paths)
 
-    async def run(self) -> None:
+    def debug_transforms(self) -> None:
+        for name in self._crawlers_to_run:
+            crawler = self._crawlers[name]
+            log.print("")
+            log.print(f"[bold bright_cyan]Debugging transforms[/] for {escape(name)}")
+            crawler.debug_transforms()
+
+    async def run(self, debug_transforms: bool) -> None:
         """
         May throw ConfigOptionError.
         """
@@ -103,6 +110,12 @@ class Pferd:
         self._load_authenticators()
         self._load_crawlers()
 
+        if debug_transforms:
+            log.output_explain = True
+            log.output_report = False
+            self.debug_transforms()
+            return
+
         log.print("")
 
         for name in self._crawlers_to_run: